Death receptors are tumor necrosis factor receptor (TNFR) gene superfamily members, of which the structural characteristics are of a cysteine-rich extracellular domain (ECD) and an intracellular death domain (DD). Tumor necrosis factor-related apoptosis inducing ligand (TRAIL) is a natural ligand for death receptors DR4 and DR5 and can activate the death receptor pathway and mitochondrion pathway of apoptotic cell death by engaging the death receptors, thereby inducing the apoptotic cell death of cells. TRAIL has a killing activity on multiple kinds of tumors while has no toxic and side effects on most normal cells. In addition, Human Immunodeficiency Virus (HIV)-infected T lymphocytes are more sensitive to the cytotoxicity effect mediated by TRAIL and the receptors thereof. These study results indicate that TRAIL and the receptors thereof can be of an attractive perspective in the treatment of cancers and AIDS.
Five natural receptors for TRAIL have been identified, including DR4 (Death Receptor 4, TRAIL-R1), DR5 (Death Receptor 5, TRAIL-R2), DcR1 (Decoy Receptor 1, TRID, TRAIL R3), DcR2 (Decoy Receptor 2, TRUNDD, TRAIL R4), and osteoprotegerin (OPG). Where, TRAIL DR4 and DR5 are capable of delivering the apoptosis signal mediated by TRAIL since they both comprise an integrate extracellular domain and intracellular domain; while decoy receptors DcR1 and DcR2 lack an intracellular domain or their intracellular domains are incomplete and can not deliver the apoptosis signal mediated by TRAIL; OPG is a secreted receptor capable of inhibiting osteoclasts and increasing bone density of which the expression in the transgenic mice model can result in a splenomegalia. OPG can inhibit the binding of TRAIL with the death receptor so as to regulate TRAIL induced apoptotic cell death. Thus, OPG is a soluble “decoy” receptor of TRAIL. In sum, all decoy receptors can compete for binding of TRAIL with DR4 and DR5 and block TRAIL-mediated apoptotic cell death.
Prior studies indicate that the expressions of death receptors DR4 and DR5 present in both human normal tissues and malignant tissues and the expression in malignant tissues is often higher. Decoy Receptor DcR1 is usually expressed in normal human tissues (transcripts thereof present in peripheral blood lymphocytes (PBL), spleen, lung, placenta, osseous tissue, prostate, thymus, testis, large intestine, small intestine and ovary), wherein the expressions in peripheral blood lymphocytes and spleen are higher, but the expressions in cancer cells and transformed cells are absent or very low; DcR2 is expressed in many normal tissues, especially the expressions in fetal liver tissues and adult testis tissues are higher, suggesting the protective role of DcR2 on these tissues. DcR2 is not expressed in the overwhelming majority of tumor cells.
Just due to the asymmetry of the distribution of death receptors in normal tissues and tumor tissues, TRAIL can specifically kill many tumor cells while have no cytotoxicity to most normal cells. Thus, it is a hot spot in the current biomedicine field to use TRAIL to induce the apoptotic cell death of tumor cells thereby specifically treating tumors. However, after many studies in depth, it is found that some cancer cell lines show resistance to TRAIL-induced apoptotic cell death, while the agonist monoclonal antibodies specifically against death receptors are more specific and safe, offering a new hope for the treatment of cancers.
The studies indicate that the monoclonal antibodies against DR4 or DR5 have intense killing activity to tumor cells while have no cytotoxicity to normal cells. The epitopes recognized by the various monoclonal antibodies or single-chain antibodies against DR4 or DR5 reported in the literature are overlapping or partially overlapping with the TRAIL binding regions on the DR4 or DR5 molecules. Thus, most of these monoclonal antibodies can compete with TRAIL for binding with death receptors DR4 or DR5.
It is reported in the literature that the extracellular region of DR5 molecule has two binding sites to TRAIL (the binding site of high affinity and the binding site of low affinity) which lies in two cysteine-rich domains (CRD1 and CRD2) respectively. Although the monoclonal antibody AD5-10 disclosed in Chinese patent number ZL02104519.4 also target the extracellular region of DR5 molecule, the binding site thereof is different from those of the monoclonal antibodies reported in the literature and the binding of AD5-10 with DR5 does not affect the binding of TRAIL with DR5. Therefore, there is a synergistic effect between TRAIL and AD5-10 when inducing the apoptotic cell death of tumor cells.